The present invention relates to preparation methods for making co-polymers that are useful in the manufacturing of future integrated circuits (“IC's”). The present invention relates to method of transport co-polymerization for making co-polymer films that can be converted into porous low dielectric (“ε”) (≦2.0) thin films. In addition, this invention relates to post treatment methods to stabilize the as-deposited co-polymer films and convert them into porous low ε (≦2.0) thin films.
During the manufacturing of IC's, multiple layers of films are deposited. Maintaining the compatibility and structural integrity of the different layers throughout the processes involved in finishing the IC is of vital importance. In addition to dielectric and conducting layers, its “barrier layer” may include metals such as Ti, Ta, W, and Co and their nitrides and suicides, such as TiN, TaN, TaSixNy, TiSixNy, WNx, CoNx and CoSi Nx. Ta is currently the most useful barrier layer material for the fabrication of future IC's that use copper as conductor. The “cap layer or etch stop layer” normally consists of dielectric materials such as SiC, SiN, SiON, silicon oxide (“SiyOx”), fluorinated silicon oxide (“FSG”), SiCOH, and SiCH. Thus, the new dielectric materials must also withstand many other manufacturing processes following their deposition onto a substrate.
Currently, there are two groups of low ε dielectric materials, which include a traditional inorganic group, exemplified by SiO2, its fluorine doped product, exemplified by FSG and its C & H doped products, SiOxCyHz, exemplified by Black Diamond and Coral respectively from Applied Materials Inc. and Novellus Inc. and newer organic polymers, exemplified by SiLK, from Dow Chemical Company. Chemical Vapor Deposition (“CVD”) and spin-on coating method have been used to deposit, respectively, the inorganic and polymer dielectric films. These current dielectric materials used in the manufacturing of the ICs have already proven to be inadequate in several ways for their continued use in mass production of the future IC's. For example, these materials have high dielectric constants (ε≧2.7), they have low yield (<5–7%) and marginal rigidity (Young's Modulus less than 4 GPa). In light of the shortcomings of current dielectric materials, a director of a major dielectric supplier has suggested that the use of thin films with high dielectric constants (e.g. ε=3.5) will be extended to the current 130 nm devices (A. E. Brun, “100 nm: The Undiscovered Country”, Semiconductor International, February 2000, p79). This statement suggests that the current dielectric thin films are at least four years behind the Semiconductor Industrial Association's (“SIA”) road map. The present lack of qualified low dielectric materials now threatens to derail the continued shrinkage of future IC's.
Currently, all conventional CVD processes have failed to make useful ε<2.7, Ta-compatible thin films. Due to many unique advantages that will be revealed in the following sections, we believe that TP soon will emerge as a primary CVD approach for fabrications of future IC's. Some of the important chemistries and mechanisms involved during TP has been reviewed previously (Chung Lee, “Transport Polymerization of Gaseous Intermediates and Polymer Crystals Growth” J. Macromol. Sci-Rev. Macromol. Chem., C16 (1), 79–127 (1977–78), pp79–127, and is hereby incorporated by reference).
Conventional CVD Processes: There are several fundamental differences between the TP and conventional CVD processes. First, in all traditional CVD processes, starting chemicals are introduced into a CVD chamber where the “feed chemicals” are subjected to needed energy sources such as plasma or ozone to generate reacting intermediates. Film will grow when these intermediates impinge onto a substrate such as a wafer. Second, in these CVD processes, wafer is normally heated and a CVD chamber is normally operated under sub-atmosphere pressure or moderate vacuum in the ranges of few mTorrs to few Torrs. Third, in these CVD processes, film not only grows on wafer but also on chamber wall. Fourth, conventional CVD processes using ozone oxidative processes are not suitable for making organic thin films. Fifth, current CVD dielectrics that are prepared from plasma polymerization of Organo-Siloxanes have ε of about 2.7 or higher.
Plasma polymerization of organic precursors can provide ε of lower than 2.7, however, they inherit many drawbacks, which include:
1. Due to poorly selective cracking of chemical bonds by plasma, some feed chemicals can end up with several reactive sites but others still have none during plasma polymerization. To avoid this disparity by increasing power levels for instance, films with highly cross-linked density and high residual stress would result.
2. During plasma polymerization, free radicals, anions, and ions with various reactive sites on each intermediate will be generated. Since intermediates with different molecular orbital configurations likely will not react with each other, some of these intermediates will have no chance to react and become a part of the resulting network. Due to this inherent complexity, plasma polymerization commonly results in poor yield (few percent) and films with different chemical structures at molecular levels.
3. Since all kinds of reactive intermediates, including very corrosive fluorine ion or radical could be generated, it is also desirable to heat the substrate, so condensation of low molecular weight products, corrosive species and not reacted impurities can be avoided. However, with presence of corrosive species such as fluorine ion, corrosion of underlying metal such as a barrier metal on wafer can become a serious problem when wafer is kept at high temperatures.
4. In addition, when more than 15 to 20 molar % of multi-functional intermediates consisting of more than two reactive sites are present inside chamber, most of these reactive sites will be trapped inside the polymer networks or become chain ends. Post annealing is done under controlled reductive or hydrogen atmosphere before the film is removed from vacuum chamber. This is needed to eliminate these reactive chain ends in order to avoid later reactions of these reactive chain ends with undesirable chemicals such as water or oxygen.
5. Finally, presence of many polymer chain-ends and pending short chains in polymer networks will result in high dielectric loss, thus the resulting dielectric will not be useful for high frequency (GHz) applications that are critical to most future IC applications. For the reasons listed above, all conventional CVD processes have failed to make useful ε<2.7, Ta-compatible thin films.
The State of Transport Polymerization: Transport polymerization (“TP”) employs known chemical processes to generate desirable reactive intermediates among other chemical species. Chemical processes that are particularly useful for this invention include photolysis and thermolysis. These two chemical processes can generate useful reactive intermediates such as carbenes, benzynes and other types of diradicals using appropriate precursors.
Photolysis can be accomplished by irradiation of compounds using electrons, UV or X-ray. However, high energetic electron and X-ray sources are expensive and typically not practical for reactors useful for this invention. When a UV photolytic process is used, a precursor that bears special leaving groups is normally required. For example, reactive intermediates such as carbenes and diradicals can be generated by the UV photolysis of precursors that bear ketene or diazo-groups. However, these types of precursors normally are expensive and not practical to use due to their very unstable nature at ambient temperatures. Other precursors and chemistry have been used for generating reactive intermediates and discussed in prior art (C. J. Lee, “Transport Polymerization of Gaseous Intermediates and Polymer Crystals Growth” J. Macromol. Sci-Rev. Macromol. Chem., C16 (1), 79–127 (1977–78), pp79–127). However, most of these precursors are quite expensive to prepare and are not readily available, thus they are not desirable nor practical for IC fabrications outlined in the current invention. A specially designed UV Reactor is used for Transport Polymerization and thin film preparation of some thermally stable precursors, and was described in a co-pending U.S. patent application Ser. No. 10/115,879 filed in Apr. 4, 2002, and entitled “UV Reactor for Transport polymerization” with Lee, et al. listed as inventors. This co-pending application is hereby incorporated by reference.
Thermolysis has been used for TP of Poly(Para-Xylylenes) (“PPX”) for the coating of circuit boards and other electronic components since early 1970s. Currently, all commercial PPX films are prepared by the Gorham method (Gorham, et al., U.S. Pat. No. 3,342,754, the content of which is hereby incorporated by reference). The Gorham method employed dimer precursor (I) that cracks under high temperatures (e.g. 600 to 680° C.) to generate a reactive and gaseous diradical (II) under vacuum. When adsorbed onto cold solid surfaces, the diradical (II) polymerizes to form a polymer film (III).

Since 1970, several commercialized products have appeared on the market with similar chemical structures. For example, a polymer PPX-D {—CH2—C6 H2Cl2—CH2—} had a dielectric constants, ε of 3.2. However, all these polymers were not thermally stable at temperatures higher than 300 to 350° C., and were not useful for fabrications of future ICs that require dielectric with lower ε and better thermal stability. On the other hand, the PPX-F, —(CF2—C6H4—CF2—)N has a ε=2.23 and is thermally stable up to 450° C. over 2.5 hours in vacuum. Therefore, rigorous attempts have been made to make PPX-F from dimer (—CF2—C6H4—CF2—)2 (Wary et al, Proceedings, 2nd Intl. DUMIC, 1996 pp207–213; ibid, Semiconductor Int'l, 19(6), 1996, p211–216) using commercially available equipment. However, these efforts were abandoned due to high cost of the dimer and incompatibility of the barrier metal (e.g. Ta) with PPX-F films prepared by TP (Lu et al, J.Mater.Res.Vol,14(1), p246–250, 1999; Plano et al, MRS Symp.Proc.Vol.476, p213–218, 1998—these cited articles are herby incorporated by reference.)
U.S. Pat. No. 5,268,202 with Moore listed as inventor (“the Moore '202 patent”), teaches that a dibromo-monomer (e.g. IV={Br—CF2—C6Cl4—CF2—Br}) and a metallic “catalyst” (Cu or Zn) inside a stainless steel pyrolyzer can be used to generate reactive free radical (V) according to the reaction (3). However, to lower the cost of starting materials, a large proportion (>85 to 95 molar %) of a more readily available co-monomer with structure {CF3—C6H4—CF3} has also been used to make PPX-F.

There are several key points that need to be addressed concerning the usage of the monomer (IV) in reaction (3). First, an earlier U.S. Pat. No. 3,268,599 (“the Chow '599 patent”) with Chow listed as inventor, revealed the chemistry to prepare a dimmer as early as 1966. However, the Chow '599 patent only taught the method to prepared dimer {CF2—C6H4—CF2}2 by trapping the diradical (V) in a solvent. Furthermore, the equipment and processing methods of the Chow '599 patent employed were not suitable for making thin films that are useful for IC fabrications. Second, according to the Moore '202 patent, the above reaction (3) would need a cracking temperature ranging from 660–680° C., without using the “catalysts”. However, we found that metallic “catalysts” such as Zn or Cu would readily react with organic bromine at temperatures ranging from 300 to 450° C., the pyrolyzer temperatures employed by the Moore '202 patent. Formation of metallic halides on surfaces of these “catalysts” would quickly deactivate these “catalysts” and inhibit further de-bromination shown in reaction (3). In addition, the presence of Zn and Cu halides inside a pyrolyzer would likely cause contamination for the process module and dielectric films on wafer. Third, cooling of reactive intermediate and wafer cooling could not be efficient because both the wafer holder and pyrolyzer were located inside a close system for the deposition chamber that was used in the Moore '202 patent. Consequently, the process module used by the Moore '202 patent cannot be useful for preparation of thin films of this invention.
Transport polymerization (“TP”) methods and processes for making low dielectric polymers that consist of sp2C—X and HC-sp3Cα—X bonds were described in a co-pending U.S. application Ser. No. 09/795,217 that was filed on Feb. 26, 2001, and entitled “Integration of Low ε thin films and Ta into Cu Dual Damascene,” with Lee, et al., listed as inventors. Wherein, X is H or preferably F for achieving better thermal stability and lower dielectric constant of the resulting polymers. HC-sp3Cα—X is designated for a hyper-conjugated sp3C—X bond or for a single bond of X to a carbon atom that is bonded directly to an aromatic moiety. Due to hyper-conjugation (see p275, T. A. Geissman, “Principles of Organic Chemistry”, 3rd edition, W. H. Freeman & Company), this C—X (X=H or F) has some double-double bond character, thus they are thermally stable for fabrications of future ICs.
However, we have observed that after Transport Polymerization, an as-deposited thin film may not achieve its best dimensional stability. Therefore, deposition conditions and post treatment methods to achieve high dimensional stability from the as-deposited films are described in a co-pending U.S. patet application Ser. No. 09/925,712, that was filed on Aug. 9, 2001 and entitled “Stabilized Polymer Film and its Manufacture,” with Lee, et al., listed as inventors. Methods to optimize the chemical stability thus achieving best electrical performance for an as-deposited film have been outlined in another co-pending application, U.S. patent application Ser. No. 10/116,724; filed on Apr. 4, 2002. In addition, after reactive plasma etching of a dimensionally and chemically stabilized film, the surface chemical composition of the resulting film has changed. Due to degradation of surface composition under reactive conditions, loss of adhesion between dielectric film and barrier metal, cap layer or etch-stop layer can occur. The invention reported herein describes processing conditions to provide good chemical stability thus interfacial adhesion between the dielectric film and the subsequently deposited top layer such as the barrier metal, the cap layer or etch-stop layer.